Chronically instrumented conscious dogs will be studied during the development of congestive heart failure due to reversible supracoronary aortic stenosis. New load-insensitive measures of left ventricular and myocardial systolic performance will be examined to identify the determinants of left ventricular dilation versus hypertrophy, to characterize the differing determinants of ventricular chamber dysfunction versus myocardial dysfunction, and to develop new methods of detecting the early onset of irreversible ventricular dysfunction. These studies should provide the pathophysiologic insight needed to develop new treatment modalities and to improve treatment timing for aortic stenosis and other forms of congestive heart failure due to left ventricular pressure overload.